1. Field of the Invention
The present invention relates to processing data obtained by an induction tool used to measure conductivity of a formation in a borehole. More particularly, the present invention relates to a method for processing signals, generated by receiver coils in the induction tool, entirely in digital form to determine the formation conductivity.
2. Description of the Related Art
The sedimentary portion of the inner surface of the earth typically includes successive layers or beds having non-uniform thicknesses. Each bed has an electrical conductivity which is indicative of the amount of hydrocarbon deposits existing in that bed. Electrical conductivity logging relates to the determination of the conductivity of the successive beds of the formation for hydrocarbon exploration. Electrical conductivity logging is based on the fact that most rocks and hydrocarbons are insulators, whereas connate waters are generally saline, and therefore, good conductors.
In geophysical well logging, a sonde or probe is lowered into a borehole in the earth. The sonde includes sensors and other equipment for measuring the physical parameters that characterize the formation. Electrical equipment forms part of the sonde for receiving and processing information from the sensors either to store data or to send the data to the surface. This data is typically sent by digital telemetry circuitry through the earth or through a wireline cable used to lower the sonde, as appropriate.
In an induction logging tool, the conductivity of the formation is measured by generating eddy currents in the formation. In general, an induction logging tool includes at least one transmitter coil and at least one receiver coil longitudinally separated and positioned along the tool axis. Induction logging measures the conductivity of the formation by first inducing eddy currents to flow in the formation in response to a current flow through the transmitter coil, and then measuring an in-phase component of a signal generated in the at least one receiver coil in response to the presence of the eddy currents. Variations in the magnitude of the eddy currents in response to variations in the formation conductivity are reflected as variations in the received signal. Thus, in general, the magnitude of the in-phase component of the received signal, that component in phase with the transmitter current as determined by a phase sensitive detector (PSD), is indicative of the conductivity of the formation.
The amplitude of the in-phase component of the signals received by the induction tool are usually derived with analog circuitry, such as that disclosed in U.S. Pat. No. 4,499,421 to Sinclair. Sinclair discloses a digital induction logging system including means for generating a plurality of transmitter frequencies. Sinclair poses the problem that prior art induction logging tools, which have been primarily analog in design, included limitations which prevented them from meeting a growing need for more precise, accurate, and error-free measurements of in-phase component signals in the received signals.
Some of the main sources of the errors and inaccuracies in measurement of phase and amplitude of the received signals are static phase-shift errors and dynamic or temperature-dependent phase-shift errors. Static phase-shift errors are those errors which occur when the tool is operating at a steady state temperature condition and generally are caused by design tolerances of electrical circuits in the tool, which include the transmitter and receiver coils, amplifiers, and PSD's. The dynamic phase-shift errors occur as a result of temperature changes occurring in the transmitter and receiver coils, the amplifiers and the PSDs. This is a major problem as great temperature differences exist at different depths in the borehole. Unpredictable phase-shifts may also be introduced by electronic component tolerances. Such phase-shift errors cause the transmitter signal to be distorted, which can cause the harmonic frequency signals of the fundamental frequency signal to have large amplitudes. Because the formation has different induction responses at different frequencies, the enhanced amplitudes of the harmonic frequency signals due to the phase shifts would introduce false signals--that is, noise--into the receiver coil, that may cause a misleading result to be obtained from the induction tool measurement.
Sinclair teaches that to obtain accurate in-phase component signal measurements that are essentially free of the static and temperature dependent phase-shift errors, a highly phase stable, low distortion transmitter signal must be generated. The Sinclair tool accomplishes this by including a waveform generator for digitally generating a low distortion, phase-stable sinusoidal transmitter signal from at least two selectable frequencies. The frequency selected can be based upon the value of the conductivity of the formations being encountered. A review of Sinclair, however, reveals that the elaborate circuitry needed to appropriately measure the formation conductivity in a borehole makes the Sinclair tool complex when the transmitter signal contains multiple frequencies. Further, the phase stable, low distortion transmitter signal does not completely remove dynamic or static phase-shift errors, thereby requiring that automatic phase compensation be provided to dynamically compensate for both the static and dynamic temperature dependent phase errors.
Another problem with analog detection circuits is that they are usually sensitive to odd harmonics of the fundamental frequency of the transmitter signal, so that there is a requirement for good spectral purity in the transmitter circuitry. This problem is addressed in U.S. Pat. No. 4,965,522 to Hazen, which discloses a multi-frequency signal transmitter with attenuation of selected harmonics, for use in an array induction logging tool. In the Hazen technique, switching and filtering circuitry is used to attenuate the amplitudes of frequency components of the third harmonic and other undesired harmonics. Thus, Sinclair and Hazen disclose the limitations associated with analog-type induction logging tools.
Therefore, it is desirable that a formation conductivity measuring technique using induction logging tools be developed that avoids the limitations of analog-type tools. Attempts at performing digital processing of the induction tool signal data have heretofore met with limited success due to the enormous amounts of data that need to be processed and transmitted by the downhole tool.